1. Field of the Invention
The present invention relates to a technology for reducing current consumption of a semiconductor integrated circuit.
2. Description of the Related Art
Recently, a power source voltage (operation voltage) of a semiconductor integrated circuit has been lowering in order to satisfy the demands for low power consumption and a decrease in gate breakdown voltage, due to miniaturization of the structure of a transistor. The operation speed of the transistor decreases as the power source voltage lowers. In order to maintain a high-speed operation of the transistor, it is necessary to lower a threshold voltage of the transistor as the power source voltage lowers.
Moreover, a sub-threshold leakage current, which passes even when a gate-to-source voltage of the transistor is set to be 0V, increases as the threshold voltage lowers. Hence, when the threshold voltage is lowered in order to maintain the high-speed operation of the transistor, a standby current of the semiconductor integrated circuit increases.
The example of disposing a switching transistor between a source electrode and a power source line of the transistor in a circuit block is disclosed in Japanese Unexamined Patent Application Publication No. Hei 5-210976. Since the switching transistor is turned off during standby, it is possible to prevent the standby current from increasing even when the threshold voltage of the transistor is lowered.
However, the above technology has the disadvantage that, when there is a latch for holding data in the circuit block, the data held in the latch is destroyed because power supply is interrupted by the switching transistor. In order to prevent the destruction of the data, it is necessary to connect the circuit block including the latch directly to the power source line, not via the switching transistor. As a result, it is impossible to prevent the sub-threshold leakage current of the circuit block like the above during standby, which causes the disadvantage that the standby current cannot be reduced sufficiently.
Meanwhile, the example of changing a substrate voltage of the transistor between a non-operation state (standby state) and an operation state of the semiconductor integrated circuit is disclosed in Japanese Unexamined Patent Application Publication Nos. Sho 60-10656 and Hei 6-89574. Specifically, the threshold voltage of the transistor is high during a non-operation period, thereby reducing the leakage current. Further, the threshold voltage of the transistor is low during an operation period, thereby heightening drivability and improving operation speed of the transistor.
However, the technology for optimally switching from the standby state to the operation state and from the operation state to the standby state has not been disclosed conventionally. When the semiconductor integrated circuit shifts from the standby state to the operation state, it is preferable to change the substrate voltage to a predetermined voltage in a short time to allow its internal circuit to be ready for the operation as soon as possible. Further, when it shifts from the operation state to the standby state, it is preferable to minimize its current consumption and change the substrate voltage to the predetermined voltage. However, such technology has not been disclosed.
It is an object of the present invention to optimally control voltages to be supplied to an internal transistor in both a standby state and an operation state in order to reduce current consumption and, more particularly, to optimally switch from the standby state to the operation state and from the operation state to the standby state, in order to reduce the current consumption.
According to one of the aspects of the semiconductor integrated circuit of the present invention, a first transistor is turned on when a circuit block including an internal transistor is in operation, to connect a substrate of the internal transistor to a first substrate voltage line. A second transistor is turned on when the circuit block is not in operation, to connect the substrate of the internal transistor to a second substrate voltage line. ON resistance of the second transistor is higher than ON resistance of the first transistor. Further, the voltage of the first and second substrate voltage lines allows a source-to-substrate voltage of the internal transistor during the non-operation of the circuit block to be higher than a source-to-substrate voltage of the internal transistor during the operation of the circuit block.
In this semiconductor integrated circuit, a substrate voltage is deep during the non-operation state so that a threshold voltage (absolute value) of the internal transistor in the circuit block is high as compared with that during the operation state. Hence, it is possible to reduce a sub-threshold leakage current and a standby current. Since the ON resistance of the second transistor is high, the substrate voltage of the internal transistor changes gradually from a first substrate voltage to a second substrate voltage when the semiconductor integrated circuit switches from the operation state to the non-operation state. Charging/discharging currents of the substrate voltage can be dispersed so that it is possible to reduce the current consumption in shifting from the operation state to the non-operation state, by which further enables reduction in the standby current during the non-operation state. Especially, a peak current in shifting from the operation state to the non-operation state can be reduced.
Meanwhile, the substrate voltage is shallow during the operation state of the semiconductor integrated circuit so that the threshold voltage (absolute value) of the internal transistor in the circuit block is low as compared with that during the non-operation state.
Hence, realizing high-speed operation of the internal transistor further enables high-speed operation of the semiconductor integrated circuit. Since the ON resistance of the first transistor is low, the substrate voltage of the internal transistor changes quickly from the second substrate voltage to the first substrate voltage when the semiconductor integrated circuit switches from the non-operation state to the operation state. The substrate voltage turns to a predetermined voltage in a short time so that it is possible to allow the circuit block to be ready for the operation quickly.
According to another aspect of the semiconductor integrated circuit of the present invention, a ratio W1/L1 between a gate width W1 and a channel length L1 of the first transistor is larger than a ratio W2/L2 between a gate width W2 and a channel length L2 of the second transistor. By differentiating the sizes of the first and the second transistors as described above, the ON resistance can be adjusted easily and accurately.
According to another aspect of the semiconductor integrated circuit of the present invention, the semiconductor integrated circuit has a plurality of circuit blocks which individually includes an internal circuit and operates independently. A plurality of first transistors and a plurality of second transistors are formed respectively corresponding to the plurality of circuit blocks. In other words, the first and second transistors are individually turned on during operation of each of the corresponding circuit blocks. The second substrate voltage is supplied via the second transistor to the substrate of the internal transistor in the corresponding circuit block being not in operation. The first substrate voltage is supplied via the first transistor to the substrate of the internal transistor in the corresponding circuit block being in operation. The substrate voltage of the internal transistor can be set according to the operation states of each circuit block as described above so that the current consumption can be further reduced.
According to another aspect of the semiconductor integrated circuit of the present invention, a third transistor connects substrates of the plurality of circuit blocks being not in operation to each other. For example, when a circuit block switches from the operation state to the non-operation state, the substrate of the internal transistor in this circuit block is connected to the second substrate voltage not only via the second transistor corresponding to this circuit block, but also via the second transistors and the third transistors corresponding to the other circuit blocks being not in operation. In other words, the second transistor corresponding to the circuit block which has already been in non-operation state is used to change the substrate voltage of the circuit block which is to be switched to the non-operation state. Thus, the second transistors can be shared by a plurality of circuit blocks, whereby the substrate voltage of each circuit block can be speedily set to a predetermined voltage.
According to another aspect of the semiconductor integrated circuit of the present invention, a power source line supplies a power source voltage to each circuit in the circuit block. A fourth transistor connects the substrate to the power source line during operation of the circuit block. For this reason, when the semiconductor integrated circuit switches from the non-operation state to the operation state, the substrate voltage of the internal transistor in the circuit block is set to a predetermined value not only via the first transistor, but also via the fourth transistor. As a result, it is possible to change the substrate voltage to a predetermined voltage in a shorter time, whereby allows the circuit block to be ready for the operation speedily.
According to another aspect of the semiconductor integrated circuit of the present invention, the gates of the first transistor and the fourth transistor are controlled by a same control signal. Hence, it is possible to reduce the number of signal lines and further the layout area of the signal lines.
According to another aspect of the semiconductor integrated circuit of the present invention, a fifth transistor is turned on when the circuit block including the internal transistor is in operation, to connect a source of the internal transistor to a first power source line. A sixth transistor is turned on when the circuit block is not in operation, to connect the source of the internal transistor to a second power source line whose voltage is lower than that of the first power source line. ON resistance of the sixth transistor is higher than ON resistance of the fifth transistor.
In this semiconductor integrated circuit, the source of the internal transistor in the circuit block being not in operation is connected to the second power source line. Hence, a source-to-drain voltage (absolute value) of the internal transistor is low as compared with that during operation. Therefore, the leakage current of the internal transistor can be reduced and the standby current can be reduced. Since the ON resistance of the sixth transistor is high, a source voltage of the internal transistor decreases gradually when the semiconductor integrated circuit switches from the operation state to the non-operation state. A current from the source of the internal transistor to the second power source line can be dispersed so that it is possible to reduce the current consumption in shifting from the operation state to the non-operation state, and to further reduce the standby current in the non-operation state. Especially, the peak current in shifting from the operation state to the non-operation state can be reduced.
Meanwhile, the source of the internal transistor in the circuit block is connected to the first power source line during operation of the semiconductor integrated circuit. Hence, the source-to-drain voltage (absolute value) of the internal transistor is high as compared with that during non-operation. Accordingly, it is possible to operate the internal transistor quickly and further the semiconductor integrated circuit quickly. Because of low ON resistance of the fifth transistor, the substrate voltage of the internal transistor increases speedily when the semiconductor integrated circuit switches from the non-operation state to the operation state. The power source voltage turns to the predetermined voltage in a short time, whereby allows the circuit block to be ready for the operation speedily.
According to another aspect of the semiconductor integrated circuit of the present invention, a ratio W3/L3 between a gate width W3 and a channel length L3 of the fifth transistor is larger than a ratio W4/L4 between a gate width W4 and a channel length L4 of the sixth transistor. By differentiating the sizes of the fifth and the sixth transistors as above, the ON resistance can be adjusted easily and accurately.
According to another aspect of the semiconductor integrated circuit of the present invention, the semiconductor integrated circuit has a plurality of circuit blocks which individually includes the internal transistor and independently operates. A plurality of fifth transistors and a plurality of sixth transistors are formed respectively corresponding to the plurality of circuit blocks. The source of the internal transistor in the circuit block being not in operation is connected to the second power source line via the sixth transistor. The source of the internal transistor in the circuit block being in operation is connected to the first power source line via the fifth transistor. The power source voltage to be supplied to the sources of the internal transistors can be set according to the operations of each circuit block as described above so that the current consumption can be further reduced.